JPH0690338B2 - Fiber type sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a sensor that is stable and highly sensitive to external environmental changes.

“Prior Art” Conventionally, as a fiber type interferometer type sensor, a Matsuhatsu Ender type sensor configured by using two optical fibers 1 and 2 as shown in FIG. 8 is known. This is one in which a sensing element 2'is provided on one of the two fibers 1 and 2 forming an interferometer, and various sensors can be obtained by changing the material of the sensing element 2'according to the object to be measured. Can be produced. For example, if polystyrene or nylon is used, the optical length of the interferometer 2 changes due to the sound wave, and thus it becomes an acoustic sensor.
When nickel or metal glass is used, distortion is generated by the magnetic field, and thus it can be used as a magnetic field sensor. Also, PV
When F ₂ or a copolymer is used, it becomes an electric field sensor, and when palladium is used, it absorbs hydrogen and expands, so it can be used as a hydrogen sensor.

[Problems to be Solved by the Invention] In FIG. 8, reference numerals 3 and 4 are optical coupling portions produced by fusing and extending two optical fibers 1 and 2. When the lengths l and h of the respective optical coupling sections 3 and 4 are adjusted to form a 3 dB coupler, the outputs of the output side fibers 1b and 2b are as follows.

However, where A is the input amplitude to the entrance side fiber 1a, φ
Is the phase change to be measured, and δ is expressed as δ = β (L ₁ −L ₂ ) (3). In equation (3), β is the propagation constant of the fiber, and L ₁ and L ₂ are the interference lengths of the fibers ₁ and ₂ . The propagation constant β is expressed as β≈kn ₁ (4) using the wave number k of light (= 2π / λ; λ is the wavelength of light in a vacuum) and the refractive index n _{1 of the} core. In the case of quartz glass, the temperature change of the refractive index Therefore, when L ₁ ≠ L ₂ , δ changes due to temperature change in the external environment, and this becomes noise during measurement.

As an example, (L ₁ −L ₂ ) = 1 cm, n ₁ = 1.4494, λ
Fig. 9 shows the result of calculation of changes in P _I and P _II when the external temperature changes ± 1 ° C when = 1.3 µm. The shaded area is the range of fluctuation. As shown in FIG. 9, both P _I and P _II greatly fluctuate with respect to a certain φ, and there is a drawback that it is very weak against fluctuations in the external environment. Next, if (L ₁ −L ₂ ) = 0 is set in order to suppress fluctuations in the external environment, a problem remains. When L ₁ = L ₂ , that is, δ = 0, the outputs P _I and P _II are as shown in FIG. 10 from the equations (1) and (2). In this case, it is stable against disturbance,
| Φ | << | However, there is a drawback that the sensitivity is very poor with respect to a slight change in φ.

It is an object of the present invention to provide a fiber type sensor which is very stable and highly sensitive to changes in the external environment, which solves the conventional drawbacks described above.

"Means for Solving the Problems" In order to achieve such an object, the fiber type sensor of the present invention has a first core in the center of the clad, and second and third eccentric positions. First and second triple-core fiber having a core, each of the first core and the second core to each other to cause optical coupling, the third core is connected so as not to cause optical coupling, In addition, each triple core fiber is provided with an optical coupling portion that is stretched to such an extent that mode coupling occurs between the first, second and third cores, and further, the second triple core fiber has a first triple core fiber. The single mode fiber having only the first core is connected to the end opposite to the side to which the core fiber is connected.

[Operation] According to the present invention, it is possible to simultaneously satisfy the disturbance resistance and the high sensitivity.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a fiber type sensor according to the present invention.
13 is a triple core fiber, 5 to 7 and 10 to 12 are cores, 16 is a single mode fiber, and 15 is a core. 9 and
Reference numeral 14 denotes an optical coupling portion, which is stretched so that optical coupling occurs between the three cores to reduce the outer diameter, and at the same time, reduce the core diameter.

As shown in FIG. 2, the triple-core fiber has three heaters 24 having three preforms having cores 17, 18, 19 and claddings 20, 21, 22 inserted in a quartz rod 23 punched with an ultrasonic drill. It can be obtained by heating at 25 ° C to 2100 ° C and drawing. In this drawn triple-core fiber, as shown in FIG. 3, each core 26, 27, 28 (hereinafter, core 26 is core I, core 27 is core II, core 28 is core II).
Call it I. ) Are arranged at intervals d so as to form an equilateral triangle. In the figure, reference numeral 2a is a core diameter, and reference numeral 2b is a fiber outer diameter.

Next, the principle of operation of the fiber type sensor thus configured will be described. First, as shown in FIG.
Core II when light of amplitude A enters the (third core) II
Consider mode coupling between (first core) and core III (second core). Regarding the mode coupling between the parallel cores in the portions other than the optical coupling portions 9 and 14 having the thin outer diameter, a. Table brew. Snyder's "Mode Coupling Theory of Optical Fibers"
(American Optical Society magazine, 62nd issue No.11, published in 1972) [AWSnyde
r "Coupled-mode theory for optical fibers" (J
our.of Opt.Soc.Am., Vol. 62, no. 11, P. 1267, 197)]. The mode coupling constant between triple cores is Given in. here Where n ₁ and n ₂ are the refractive indices of the core and the clad. FIG. 4 shows the normalized mode coupling coefficient using equation (7). Calculate the relationship between (a is the core radius, Δ is the relative refractive index difference) and d / a obtained by normalizing the distance d between the cores of the triple core fiber shown in the inset diagram with the core radius a, and using v as a parameter. It was drawn. The inset of FIG. 4 shows the positional relationship between the three cores of the triple-core fiber, showing the core spacing d, the core radius a, the core refractive index n ₁ and the clad refractive index n ₂ . It is a thing. FIG. 4 is a diagram for estimating a core radius a that causes mode coupling between cores.
When light of amplitude A enters the core I and the fiber length is 1, the light intensity of the cores I, II, and III is Given in. When l = about 10 m, optical coupling does not substantially occur except in the optical coupling sections 9 and 14, that is, P _II and P _III are equal to or less than −50 dB with respect to P _I. Therefore, k must be 3.2 × 10 ⁻⁴ (m ⁻¹ ) (14). From FIG. 4 showing the relationship between the mode coupling coefficient k and the standard distance d / a between the cores, Δ = 0.3%, a = 5
It can be seen that d / a12 is required when μm and v = 2.2. That is, if d / a = 12, the optical coupling between the three cores is small enough to be neglected except for the optical coupling portions 9 and 14.

In this case, the radius of the core is about 1/2 (that is, v is 2.2 to 1.
The main purpose is to explain that the mode coupling occurs between the cores if the fiber is drawn so that it changes to 1). Therefore, the case of λ = 1.55 μm is assumed, but the designation of the wavelength is not particularly important.

Next, as shown in FIG. 1, two triple-core fibers 8
Since the cores II and III are connected so that only the cores II and III cause optical coupling and the core I does not cause optical coupling, the fiber type sensor of this embodiment is equivalently represented as shown in FIG. You can In the optical coupling parts 9 and 14, the core diameter is 2a more than other parts.
Since v is small, the v value (see equation (9)) is small and the mode coupling coefficient K is large. For example, if the outer diameter of a triple-core fiber is extended to about 1/2, d / a = 12
Does not change, but the v value becomes about 1.1. At this time, the mode coupling coefficient k becomes K = 20 m ⁻¹ (15) from FIG. 4, and the mode coupling sufficiently occurs when the coupling length is about 1 cm. In FIG. 5, when the light having the amplitude A is incident on the core I, the intensity of the light emitted from the cores I, II, and III on the emission side is represented by the following equation.

Here, φ is the phase change to be measured, and the mode coupling coefficient at the optical coupling part is It is made so that

FIG. 6 shows the dependence of P _I , P _II and P _{III on} the phase change φ. In this embodiment, P _II is measured, but the sixth
As you can see from the figure, for | φ | << | Therefore, it can be seen that the sensitivity is sufficiently high even for a small φ.

FIG. 7 shows an example in which the optical fiber type sensor of this embodiment is applied as a hydrogen detector. In FIG. 7, 31 and 31 are triple core fibers, 32 is a single mode fiber, 33 and 34 are optical coupling parts, 35 to 37 are cores, and 38 is palladium coated on the triple core fiber. Note that 39 and 40 are fiber connecting portions.

Palladium expands when it comes into contact with hydrogen. The light passing through the cores II and III in the triple-core fiber undergoes different phase changes due to the expansion of palladium because the positions where the cores are arranged are different. If this difference is φ, the output light P _II of the single-mode fiber changes its light intensity with the change of φ, as given by the equation (17). The present hydrogen detector can detect even a small φ (that is, the amount of hydrogen) with high sensitivity. Further, since palladium releases hydrogen and returns to the original state when heated, the hydrogen detector can be repeatedly used.

[Advantages of the Invention] As described above, according to the present invention, the first and second triples having the first core at the center of the cladding and the second and third cores at the eccentric positions are provided. The core fibers are connected so that each of the first cores and the second cores cause optical coupling and the third cores do not cause optical coupling, and each triple core fiber has a first , An optical coupling part extended to the extent that mode coupling occurs between the second and third cores, and further, the second triple core fiber is opposite to the side to which the first triple core fiber is connected. Since the single mode fiber having only the first core is connected to the end portion, it is very stable against the external environment change and it is possible to detect a minute change of the measuring object. Further, since the characteristics of the optical fiber do not change even when bent to a bending diameter of about 3 cm, the fiber type sensor of the present invention has a great feature of being highly flexible. In addition, in particular, since the first core of the second triple-core fiber and the core of the single-mode fiber are optically coupled, the light intensity of the light emitted from the first core of the second triple-core fiber to the right ( There is an effect that only the light intensity (P _II ) of one core can be detected. That is, the light intensity (P _I ) of the third core
And it is not necessary to detect the luminous intensity (P _III ) of the second core,
On the contrary, it causes noise. A reliable method for detecting only the light intensity (P _II ) of the first core is to optically splice a single mode fiber having only the first core.

[Brief description of drawings]

1 to 7 show an embodiment of the present invention, FIG. 1 is a schematic perspective view, FIG. 2 is a schematic perspective view showing a method for producing a triple core fiber, and FIG. 3 is a crossing of the triple core fiber. Fig. 4 is a graph showing the calculated values of the mode coupling coefficient of the triple core fiber, Fig. 5 is an equivalent circuit diagram of the fiber type sensor of Fig. 1, and Fig. 6 is the output of the fiber type sensor of Fig. 1. FIG. 7 shows a change in light intensity, FIG. 7 is a schematic perspective view showing an example in which the fiber type sensor of the present invention is used as a hydrogen detector, FIG. 8 is a schematic plan view of a conventional interference type fiber sensor, and FIG. FIG. 10 is a diagram showing an output variation of the conventional fiber sensor, and FIG. 10 is a diagram showing an output variation of the conventional fiber sensor. 1,2 …… Single mode fiber, 2 ′ …… Sensing element, 3,4 …… Optical coupling part, 1a, 2a …… Injection side fiber,
1b, 2b …… Emitting side fiber, 5,6,7 …… Core, 8,13 …… Triple core fiber, 9,14 …… Optical coupling part, 10,11,12 ……
Core, 16 ... Single mode fiber, 15 ... Core, 17,18,
19 …… core, 20,21,22 …… clad, 23 …… quartz jacket, 24,25 …… heater, 26,27,28 …… core, 29 …… clad, 30,31 …… triple core fiber, 32 …… Single mode fiber, 33,34 …… Optical coupling part, 35,36,37 …… Core, 38 …… Palladium, 39,40 …… Connecting part.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location G02B 6/02 A 7036-2K 6/28 W 8707-2K

Claims (1)

[Claims] 1. An interferometric sensor using an optical fiber, wherein a first core is provided at the center of the clad and second and third cores are provided at eccentric positions of the clad. The first core and the second core are connected so that the first core and the second core are optically coupled with each other and the third cores are not optically coupled with each other. An optical coupling part that is stretched to the extent that mode coupling occurs between the second and third cores is provided, and the second triple core fiber is provided at the end opposite to the side to which the first triple core fiber is connected. A fiber type sensor, wherein a single mode fiber having only a first core is connected.